Gas-insulated high-voltage semiconductor valve means

ABSTRACT

A gas-insulated semiconductor valve for high voltage power has an elongated valve stack with a plurality of semiconductor elements. The valve stack is provided with electrostatic shields for reducing the stresses on the insulating medium. The shields comprise a plurality of annular shields distributed along the longitudinal axis of the valve stack. The shields are arranged in planes which are substantially perpendicular to the longitudinal axis of the stack and surround the valve stack. The curved part of each shield has a substantially constant radius of curvature.

TECHNICAL FIELD

The present invention relates to a gas-insulated semiconductor valvemeans for high voltage and for high power, which comprises a stack ofvalves with a plurality of semiconductor elements electricallyseries-connected between a first and a second electrical mainconnection. The valve stack has a longitudinal axis and the mainconnections are arranged at opposite ends of the longitudinal axis ofthe stack. The valve stack is provided with electrostatic shields forreducing the stresses on the insulating gas.

The concept "high voltage" as used in this application relates to avalve voltage exceeding about 50 kV, and the concept "high power"relates to a rated power exceeding about 100 MW in a twelve-pulseconverter with valves according to the present invention.

The concept "main connection" as used in this application means aconnection to a valve which is intended to carry the load current of thevalve, this to distinguish from other electrical connections to thevalve which may be arranged for, for example, control and measurementpurposes.

BACKGROUND OF THE INVENTION

Semiconductor valves of the kind mentioned in the introduction arepreviously well-known. The semiconductor elements may consist ofthyristors or other controllable semiconductor elements, or of diodes.Such valves are used within electric power engineering in powertransmission plants. An important field of use is as valves inconverters in installations high voltage power transmission. Anotherfield of use of the valves is as control and switching means inequipment for series- or parallel compensation in ac networks.

Installations of the kind referred to here often have very highoperating voltages. The valve voltages often lie at one or a few hundredkV, and the operating voltages relative to ground may be in the interval500-1000 kV. A result of this is that large insulation distances arerequired, and the valve and the equipment have large dimensions andrequire large space.

The above-mentioned disadvantages are especially prominent in the caseof enclosed, gas-insulated valves designed for outdoor erection. From,for example, the International Patent Applications WO 93/17488 and WO95/28030, such valves are known. Each valve (possibly half a valve ortwo series-connected half valves) is arranged in a separate enclosure.The valve is gas-insulated, and the housing in which the valve itself isarranged is filled with a suitable gas, for example air, nitrogen, orSF₆ (sulphur hexafluoride).

Valves of the above-mentioned kind have considerable advantages. Theenclosures with the semiconductor valves mounted therein may, inprinciple, be prepared completely at the factory, and the need of thelarge valve halls is completely eliminated. However, in case of valvesfor higher voltages, the completion of the valves at the factory entailsrelatively large transport dimensions. At the highest currentlyoccurring voltages, the dimensions of the enclosures would be so largethat they could not be accommodated within conventional loading-gaugesand thus could not be transported at all. The large dimensions of theenclosures would also entail a relatively large need of ground area forerection of the valves, usually 12 or 24 valves, which are included in asingle-pole and a two-pole HVDC converter station, respectively.

SUMMARY OF THE INVENTION

The invention aims to provide a semiconductor valve means of the kinddescribed in the introductory part of the description, which hasconsiderably smaller dimensions than prior art semiconductor valves andwhich thus requires less ground area or hall space for their erection.

The invention preferably aims to provide an enclosed semiconductor valvefor outdoor erection, the dimensions of which, also at the highestvoltages occurring, will not be so large as to prevent transportationthereof by conventional means of transport.

In a means according to the invention, a plurality of electrostaticshields are arranged one after another along the stack of valves. Eachshield is annular and surrounds the stack of valves. The shields arepreferably in the form of profiles curved as a circular arc with aconstant radius of curvature. The potentials of the shields arecontrolled so as to approximately follow the potential of adjacent partsof the valve stack. This makes possible considerably shorter insulationdistances between the valve stack and adjacent objects than what hasbeen possible so far.

In a preferred embodiment, the valve means is enclosed, one end of thevalve stack being positioned at or near the potential of the enclosureand the other end thereof being connected to a bushing. Both the valvestack and the bushing are surrounded by shields, the potentials of whichare controlled so that the voltage between the shields and the housingis greatest at the connection of the valve stack to the bushing anddecreases in both directions. It has been found that in this way such aconsiderable reduction of the dimensions of the valve means can beobtained that, also at the highest voltages occurring, the valves meansmay be completed at the factory and be transported as finished units tothe sites of erection.

In an embodiment which, from a mechanical and an electrical point ofview, is particularly simple and advantageous, both the valve stack andthe bushing are horizontal and arranged one after another in thelongitudinal direction of the elongated housing.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail in the following withreference to the accompanying drawings, wherein.

FIG. 1 shows a longitudinal section through a valve means according tothe invention;

FIG. 2 shows an example of the embodiment of the valve stack itself inthe means according to FIG. 1;

FIG. 3 shows a cross section through the means according to FIG. 1;

FIG. 4 shows in more detail the embodiment of one of the shields of themeans;

FIG. 5a shows how the potential control of the shields can be obtainedby connecting the shields to suitable points on the valve stack;

FIG. 5b shows how, according to an alternative embodiment, a separatevoltage divider can be arranged for potential control of the shields;

FIG. 6 shows an example of how the potentials of the shields vary withtheir position along the valve stack and the bushing; and

FIG. 7 shows an alternative embodiment in which the valve stack isarranged suspended from its housing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a longitudinal section through a valve means according tothe invention, that is, the longitudinal axis of the valve stack lies inthe plane of the paper and is horizontal in the figure. The valve meansis intended for outdoor erection with the orientation shown in thefigure, that is, with the longitudinal axis of the valve stacksubstantially horizontal. Of the valve means, the figure shows only thehousing 1 and the valve stack 2 (in dashed lines) with their bushingsand electrostatic shields.

The housing 1 comprises a metal housing adapted for outdoor erection,which in a known manner is provided with means (not shown) for, forexample, ventilation, pressure-maintenance, temperature control and forcooling of the valve stack as well as for communication with the valvestack in the form of control, measurement and monitoring signals. In theexample shown the housing is filled with air of atmospheric pressure,and the air constitutes the insulation medium which takes up the voltagebetween the housing and the valve stack. The housing may be at groundpotential or, which is often the case in, for example, converter valves,at a potential which deviates from ground potential from the points ofview of both direct and alternating voltage.

The valve stack 2 is schematically shown in the figure in dashed lines.At its two ends, defined by the longitudinal axis, it has its mainconnections 23 and 24. Between these connections, a plurality ofsemiconductor elements, for example 50-250, are connected in series, andthese elements may, for example, consist of thyristors. In a knownmanner, the valve stack comprises means for mounting, for cooling andfor protection of the elements against, for example, over-voltages, aswell as voltage dividers and control, measurement and monitoringcircuits. The valve stack is supported by support insulators, arrangedon the floor of the housing 1, of which the insulators 9_(1a) -9_(12a)are shown in the figure.

The main connection 24 of the valve stack lies at a potential which, inprinciple, is the same as the potential of the housing 1. However, via abushing 4 for low voltage, the connection is passed through the wall ofthe housing. A surge arrester.for low voltage may be arranged betweenthe housing and the connection 24 to ensure that the connection does notdeviate from the potential of the housing by more than a limitedabsolute value. The main connection 23 of the valve stack is passed, viaa high-voltage bushing 6, through the housing wall. A surge arrester 12shown in dashed lines is arranged between the connection 23 and thehousing 1.

According to the invention, a number of identical annular electrostaticshields 7₁ -7₂₉ are arranged along the valve stack. Each shield lies ina plane which is perpendicular to the longitudinal axis of the stack,and the shield surrounds the stack. Each shield, for example 7₁,comprises an upper part, 7_(1a), and an lower part, 7_(1b).

In the same way, a number of identical shields 8₁ -8₈ are arranged alongthe bushing 6. These shields are identical with the shields of the valvestack and, like these, are arranged in planes which are perpendicular tothe longitudinal axis of the stack. These shields surround both thatpart of the bushing 6 which is situated inside the housing 1 and thesurge arrester 12.

FIG. 2 shows two of the six sections of the valve stack. Each of theseis supported by four support insulators, of which the insulators 9_(3a),9_(4a), 9_(5a) and 9_(6a) are shown. Each section comprises twothyristor modules, 20₃, 20₄, 20₅ and 20₆, and two reactors, 21₃, 21₄,21₅ and 21₆. The reactor modules and the upper thyristor modules aresupported by support insulators, of which the insulators 10_(3a)-10_(6a) and 11_(3a) -11_(6a) are shown. The electrical seriesconnection of the thyristor modules and the reactors is made in the wayshown in the figure by means of connection conductors, for example 22,schematically shown in thick lines.

FIG. 3 shows, viewed from the left in FIG. 2, the valve stack sectionwhich has the thyristor modules 20₃ and 20₄. The figure shows thethyristor modules, supported by the support insulators 9_(3a), 9_(3b),10_(3a), 10_(3b), 11_(3a), 11_(3b), and the reactor 21₄. The figure alsoshows one of the shields surrounding the valve stack, namely, the shield7₁ consisting of the two identical parts 7_(1a) and 7_(1b). Each shieldpart comprises a light-metal profile which is curved so as to form acircular arc with a center angle which is somewhat large than 180°. Theother shields of the valve stack are identical with that shown in FIG.3.

Since each shield comprises two parts, the shield can be mounted anddismantled independently of the other shields, which is a significantadvantage both during the manufacture and during the supervision work onthe valve stack. Since each shield part forms a circular arc, the centerangle of which exceeds 180°, the joints or the transitions between thetwo shield halves are somewhat retracted towards the valve stack. Inthis way, the external field strength is reduced at these portions,which increases the electrical strength of the means. The shields cansuitably be mounted to the valve stack by means of suitable metallic orelectrically insulating rod or other means of attachment (see furtherbelow with reference to FIG. 5).

FIG. 4 shows a vertical section through the longitudinal axis of thevalve stack through the upper shield part 7_(1a). The shield is madefrom a symmetrical light-metal profile with a smoothly rounded crosssection. In the embodiment shown here, its external envelope surface 73forms an angle α with the direction L--L of the longitudinal axis of thestack. The angle α may, for example, be about 10°, and the inclinationis such that the distance of the envelope surface from the valve stackdecreases from left to right in FIG. 1, that is, from that end of thestack which has the highest voltage relative to the housing 1 to thatend which has the lowest voltage relative to the housing. All theshields 7₁ -7₂₉ are made in this way. In this way, the envelope surfacesof the shields will follow the electrical equipotential surfaces moreclosely, which contributes to reduce the field strength outside theshield.

FIG. 5 shows two alternative ways of controlling the potentials of theshields 7. Their potentials are controlled so that they approximatelyfollow the potential of the valve stack, that is, so that they have thehighest voltage relative to the housing at the main connection 23 of thevalve stack and that their voltage decreases along the valve stack in adirection towards the second main connection 24 thereof. FIG. 5a showshow some shields 7_(n), 7_(n+1), 7_(n+2), 7_(n+3) with the aid ofelectrically conducting means 71_(n), 71_(n+1), 71_(n+2), 71_(n+3) areconnected to schematically shown connection points 72_(n), 72_(n+1),72_(n+2), 72_(n+3) on the valve stack. This consists of a relativelylarge number of series-connected semiconductor elements, and all ofthese are normally accessible to electrical connection. It is thereforepossible to choose suitable connection points for the different shieldsso that the desired potential distribution is obtained, for example apotential which decreases linearly along the stack from the shield 7₁ tothe shield 7₂₉, which has a low voltage relative to housing. The chosenconnection points thus need not, as in the figure, be positioned rightin front of the shields but may be displaced to a certain extentrelative thereto in one or the other direction in the longitudinaldirection of the stack.

The electrically conducting means 71_(n) etc. may consist ofelectrically conducting mounting rods or similar means, by means ofwhich the shields are fixed to the valve stack. Alternatively, ofcourse, the shields may be mounted to the valve stack with the aid ofelectrically insulating mounting means, in which case separateconductors are used for connection of the shields to their connectionpoints on the valve stack.

FIG. 5b shows how a separate voltage divider can be used for control ofthe potentials of the shields. The voltage divider is connected betweenthe two main connections 23 and 24 of the valve stack and comprises anumber of sections, each one with one connection point for each shield.Each section has one resistive-capacitive branch (resistors R1_(n),R1_(n+1), R1_(n+2) and the capacitors C_(n), C_(n+1), C_(n+2)) fordynamic voltage distribution and one purely resistive branch (resistorsR2_(n), R2_(n+1), R2_(n+2)) for static voltage division. The figureshows as an example the three sections of the voltage divider which hasthe connection points for the shields 7_(n), 7_(n+1), 7_(n+2), 7_(n+3).In this case, the connection conductors 71_(n) -71_(n+3) of the shieldsare connected to the voltage divider in the way shown in the figure, andthe voltage divider controls the potentials of the shields. Also in thiscase the desired potential distribution may be obtained by a suitablechoice of impedances of the components of the voltage divider sections.In this case the shields are electrically insulated from the valvestack, for example mounted thereto with the aid of electricallyinsulating mounting means. Further, the embodiment according to FIG. 5bhas the advantage that, by a suitable choice of capacitances of thecapacitors of the voltage divider sections, stray capacitances betweenthe shields and the housing can be compensated for so that the desiredvoltage distribution is obtained also in case of rapid transients. Forlimitation of the voltage between the valve stack and the shields/thevoltage divider, arresters Z_(n), Z_(n+1), Z_(n+2), Z_(n+3) are arrangedbetween the voltage divider and the valve stack, whereby the risk ofelectrical flashovers, arising under certain abnormal operatingconditions, is eliminated.

Alternatively, the resistors R2_(n), R2_(n+1), R2_(n+2), etc., can bereplaced by or be supplemented with metal-oxide varistors, which thenalso ensure harmless values of the voltages between shields positionedadjacent to each other.

The shields 8₁ -8₇ are identical with the shields 7₁ -7₂₉. However, theyare mounted so that their envelope surfaces incline in oppositedirections to the corresponding surfaces of the shields 7₁ -7₂₉, thatis, the distance of each envelope surface from the valve stack decreasesfrom right to left in FIG. 1. The potentials of the shields arecontrolled in the same way as the potentials of the shields 7₁ -7₂₉,that is, either so that they are connected to suitably located points onthe valve stack 12 (in similar manner as that shown in FIG. 5a) or thata separate voltage divider connected between the housing 1 and theconnection 23 is arranged for the potential control of the shields (insimilar manner as that shown in FIG. 5b).

FIG. 6 shows how the potentials of the shields vary with their positionalong the valve stack and the bushing. The distance of the shields fromthe lefthand end wall of the housing 1 in FIG. 1 is designated x, thatis, at the left-hand end wall of the housing x=0, and at the righthandend wall of the housing x=L, where L is the length of the housing. Atthe lefthand end of the valve stack, that is, at the main connection 23,x=a. The potentials of the shields are designated U_(s), the potentialof the housing being 0. According to a preferred embodiment, thepotentials of the shields are controlled according to the curvedesignated A, that is, the voltage between the shields and the housingis highest at the lefthand end of the valve stack and decreasesapproximately linearly towards the two ends of the housing.

In another preferred embodiment, the potentials of the shields arecontrolled so that a certain minor part, for example about 10%, of thevoltage between the housing and a certain point of the valve stack istaken up between the stack and the shield positioned outside this point,whereby the voltage stress between the shields and the housing can bereduced. One example of the potential distribution in this embodiment isshown by the curve designated B.

The two potential distributions shown in FIG. 6 may, of course, beadjusted to a greater or smaller degree depending on the requirementsand the circumstances of the particular case, and the most suitablepotential distribution in a certain case may, in principle, bedetermined with the aid of three-dimensional field calculations.

FIG. 7 shows a cross section through a valve means according to analternative embodiment of the invention. The schematically shown valvestack 2 with its shields 7_(ia) and 7_(ib) is here arranged suspendedfrom the housing 1 and is supported by suspension insulators, which areattached to the upper part of the housing 1, and of which the insulators9_(ja) and 9_(jb) are shown.

It has proved that the transverse dimensions (width and height) of avalve means according to the invention can be reduced drastically (in atypical case, both width and height were approximately halved) comparedwith what would have been required in a corresponding prior art valvemeans. Also at the highest operating voltages occurring, an enclosedoutdoor valve according to the invention will therefore have suchmoderate dimensions that it can be transported without problems from thefactory to the site of erection, and, in addition, the need of groundarea for an installation with valve means according to the inventionwill be considerably smaller than for prior art valve means of acorresponding kind. The reduced dimensions also entail a reduction ofthe weight and cost of a valve means.

In the foregoing, an enclosed valve means designed for outdoor erectionis described. The invention can also be applied to a non-enclosed valvedesigned for outdoor erection, in which case the good utilization by theinvention of the insulating properties of the air makes possible aconsiderably reduced distance between the valves and between these andother apparatus, which results in a smaller volume and lower cost ofvalve halls.

In the valve means described above, air is used as insulating medium,and the housing is filled with air having atmospheric pressure.Alternatively, other gaseous insulating media may be used, for examplenitrogen or SF₆, and then possibly at higher pressures than atmosphericpressure.

In the foregoing, a valve means is described which has one single valvein the strict sense of the word. However, the invention can also beapplied, with appropriate changes, to valve means which in an enclosurehas, for example, one part of a valve, or two valves series-connected toeach other, or adjoining parts of two valves.

In the valve means described above, the low-voltage part (the mainconnection 24) of the valve stack is connected to a bushing.Alternatively, however, this connection can be directly connected to thehousing or enclosure of the means.

The embodiment of the electrostatic shields described above is only anexample of a preferred embodiment.

The shields described above are made of profiles bent as a circular arc,that is, the radius of curvature is constant and hence within the scopeof given external dimensions as large as possible, which gives the leastpossible stresses on the insulating medium. In order that the advantagesof the invention, in the form of reduced dimensions of the valve means,shall be as large as possible, the dimensions of the shields, in a planeperpendicular to the longitudinal axis of the stack, should be as smallas possible. This means that the radius of curvature of the shieldshould be of the same order of magnitude as, and exceed by as small anamount as possible, the transverse dimensions of the valve stack.Another factor which must also be taken into consideration is whethereach shield is arranged at approximately the same potential as parts ofthe valve stack located inside the shield, or whether the shields, inthe manner described above, are given such potentials that a certainminor part of the valve voltage is taken up between the stack and theshields.

It is, of course, within the scope of the invention to design theshields with a different shape, for example with a somewhat varyingradius of curvature. Also, the profile of the shields may deviate fromthe symmetrical profile described above with an inclined externalenvelope surface, and the profile may be unsymmetrical and/or have anexternal envelope surface which is substantially parallel to thelongitudinal axis of the valve stack.

The two-part shields described afford significant advantages duringmounting/dismantling, but, alternatively, of course, each shield may bedesigned in one single part, and for example, have the shape of onesingle, closed circular arc, or be designed as two semicircular arcsjoined by more slightly curved (possibly straight) portions. Further,within the scope of the invention, the shields may be designed in morethan two parts. However, the shape described above affords considerableadvantages in the form of simple manufacture (all the shields halves areidentical with each other), simple mounting/dismantling and a shapewhich gives least possible electrical stresses on the insulating medium.

In the embodiment described above, the shields are identical andequidistant, but if considered appropriate, the scope of the inventionpermits the shields to have different widths and/or to be placed atdistances from each other which vary along the longitudinal axis of themeans, for example with the shields located more spaced from each othernearer the ends of the means, where the voltage stresses are lower.

The methods for control of the potentials of the shields described abovemay be varied within the scope of the invention. One possible way ofcontrolling the potentials of, for example, the shields located outsidethe high-voltage bushings (6 in FIG. 1) is to use the cooling waterpipes of the valve as voltage divider. The pipes are then provided withelectrodes connected to the shields, which electrodes are placed so asto obtain the desired potential distribution of the shields.

What is claimed is:
 1. A gas insulated semiconductor valve means forhigh voltage and for high power, comprising a valve stack with aplurality of semiconductor elements electrically series-connectedbetween a first and a second electrical main connection, and anenclosure, surrounding the valve stack, with an elongated bushingarranged in the wall of the enclosure, the valve stack having alongitudinal axis (A-A) and connections arranged at the opposite endsthereof, a first connection at low potential relative to the enclosureand a second connection connected to the bushing, the valve means havinga first set of shields, said set having a plurality of annularelectrostatic shields distributed along the longitudinal axis of thevalve stack arranged in a plane substantially perpendicular to thelongitudinal axis, and surrounding the valve stack, each shield withcurved paths, so as to be of annular shape, whereby the curved parts ofeach shield have a substantially constant radius of curvature wherebythe shields in the first set of shields are electrically connected to afirst potential-controlling member to control the potentials of theshields so that the voltages between the shields and the surroundings ofthe shields successively diminish along the valve stack in a directionaway from the second connection towards the first connection, andwherein the semiconductor valve means has a second set of shields, saidsecond set of shields having a plurality of electrostatic shields,distributed along the bushing between the second connection and theenclosure, and surround the bushing.
 2. A valve means according to claim1, wherein the first potential-controlling member comprises a voltagedivider connected in parallel with the valve stack and having terminalselectrically connected to the shields.
 3. A valve means according toclaim 2, wherein the voltage divider is composed of resistive andcapacitive elements.
 4. A valve means according to claim 1, wherein thefirst potential-controlling member comprises the valve stack, which isprovided with terminals electrically connected to the shields.
 5. Avalve means according to claim 1, wherein the firstpotential-controlling member is adapted to impart to at least certainshields in the first set of shields potentials which are closer to thepotential of the first connection than the potentials of those parts ofthe valve stack located in the same plane as the respective shield.
 6. Avalve means according to claim 1, wherein the bushing has a longitudinalaxis substantially parallel to the longitudinal axis of the valve stack,wherein each shield in the second set of shields is arranged in a planesubstantially perpendicular to the longitudinal axes of the bushing andthe valve stack, and the curved parts of each shield have asubstantially constant radius of curvature.
 7. A valve means accordingto claim 6, wherein the shields in the second set of shields aresubstantially identical with the shields in the first set of shields. 8.A valve means according to claim 1, wherein the shields in the secondset of shields are electrically connected to a secondpotential-controlling member adapted to control the potentials of theseshields so that the voltage between the shields and the enclosuresuccessively diminishes along the bushing in a direction away from thesecond connection towards the enclosure.
 9. A valve means according toclaim 1, wherein a valve arrester is arranged parallel to the bushingbetween the second connection and the enclosure, and whereby the shieldsin the second set of shields are adapted to surround both the bushingand the arrester.
 10. A valve means according to claim 9, wherein asecond potential-controlling member comprises the surge arrester, whichhas terminals electrically connected to the shields in the second set ofshields.
 11. A valve means according to claim 8, wherein the secondpotential-controlling member comprises a separate voltage divider.
 12. Avalve means according to claim 8, wherein the secondpotential-controlling member is adapted to impart to at least certainshields in the second set of shields potentials which are closer to thepotential of the enclosure than to the potential of those parts of thevalve means which are surrounded by the respective shield and located atthe same plane as the shield.
 13. A valve means according to claim 1,wherein at least certain of the shields are divisible to facilitate themounting and dismantling of the shields.
 14. A valve means according toclaim 13, wherein each divisible shield comprises two parts, wherebyeach part is formed substantially as a circular arc with a constantradius.
 15. A valve means according to claim 14, wherein the parts of adivisible shield, where the two shield parts adjoin each other, areretracted in a direction towards the valve.
 16. A valve means accordingto claim 14, wherein each of the two parts of a shield is substantiallyformed as a circular arc with a center angle which is larger than 180 °.17. A valve means according to claim 1 wherein at least certain shieldsare arranged with external envelope surfaces which are inclined inrelation to the longitudinal axis of the valve stack so that thedistance between the envelope surface and the valve stack diminisheswith an increasing distance from the second connection.